What Is a Molecular Sieve?

A molecular sieve is a manufactured crystal used to separate molecules of a particular size from a gaseous of liquid mixture. Commonly used in chemical manufacturing and petroleum refining, molecular sieves can be used multiple times with little change or degradation of the crystal structure. Molecular sieves are made to customer specifications and come in different sizes and porosities.

History and Manufacturing

The molecular sieve was invented and first patented by the research and development labs of Union Carbide in the 1950s. As of 2011, molecular sieves are manufactured in the United States as well as Italy, Japan and China.

Chemical Composition

Molecular sieves are made of zeolite. According to Encyclopedia Brittanica, zeolite comes from the family of "hydrated aluminosilicate minerals that contain alkali and alkaline-earth metals." Zeolite is valued because it can be readily manipulated into artificial crystals with uniform and well-defined pore size.

How They Work

A molecular sieve works like a sponge. When heated, molecular sieves readily dehydrate and shed all water content with little or no damage to the crystal structure. This property means the molecular sieve can be reused many times before it must be replaced. When added to a gas or liquid mixture, the dehydrated sieve absorbs not only water, but also other molecules that fit into the pores on the surface. This action not only dries the mixture, but also removes selected molecules depending on the pore size.

Applications

Molecular sieves are used in everything from heavy manufacturing to laboratory work. For example, J.T. Baker Chemical recommends a type 3A molecular sieve for removing atmospheric water from acetone-based solvents before using them to prepare samples for electron microscopy. Water molecules are much smaller than acetone and will be readily absorbed into the sieve. Conversely, the natural gas industry uses large-scale molecular sieves of varying porosities to remove water, mercury, sulfur and carbon dioxide from raw natural gas. Molecular sieves are so sensitive and specialized that they are also used to remove unwanted odors and tastes from food and beverage products.